Shared floating-point registers and register port-pairing in a dual-architecture CPU

ABSTRACT

A dual-instruction-set central processing unit (CPU) is capable of executing floating point instructions from a reduced instruction set computer (RISC) instruction set and from a complex instruction set computer (CISC) instruction set. Floating point data is transferred from a CISC program to a RISC program running on the CPU by using shared floating point registers. The architecturally-defined floating point registers in the CISC instruction set are merged or folded into some of the architecturally-defined floating point registers in the RISC architecture so that these merged registers are shared by the two instructions sets. In particular, the floating-point exception-mask and flags registers defined by each architecture are merged together so that CISC instructions and RISC instructions implicitly update the same merged flags register when executing floating point instructions. The RISC and CISC registers are folded together so that the CISC flags and RISC flags with the same function are merged to the same register bit. The floating-point data registers are also merged together, allowing a CISC program to pass floating-point data to a RISC program merely by writing one of its floating-point data registers, switching control to the RISC program, and the RISC program reading one of its floating-point data registers that is merged with and corresponds to the CISC floating-point data register that was written to by the CISC program. An extended-precision CISC data format is supported by pairing two of the RISC-size floating-point data registers.

BACKGROUND OF THE INVENTION--RELATED APPLICATION

This application is a continuation-in-part of application for a "Shared Register Architecture for a Dual-Instruction-Set CPU", filed Jul. 20, 1994, U.S. Ser. No. 08/277,962, now U.S. Pat. No. 5,481,693, hereby incorporated by reference. This related application has a common inventor and is assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION--FIELD OF THE INVENTION

This invention relates to computing hardware, and more particularly to the architecture of floating point registers in a processor capable of executing from two instruction sets.

BACKGROUND OF THE INVENTION--DESCRIPTION OF THE RELATED ART

Processors, or central processing units (CPU's) that are capable of executing instructions from two separate instruction sets are highly desired at the present time. For example, a desirable processor executes user applications for the x86 instruction set and the PowerPC™ instruction set. It is able to execute the tremendous software base of x86 programs that run under the DOS™ and WINDOWS™ operating systems from Microsoft of Redmond, Wash., and it could run future applications for PowerPC™ processors developed by IBM, Apple, and Motorola.

Such a processor is described in the related copending application for a "Dual-Instruction-Set Architecture CPU with Hidden Software Emulation Mode", filed Jan. 11, 1994, U.S. Ser. No. 08/179,926. That dual-instruction-set CPU has a pipeline which is capable of executing instructions from either a complex instruction set computer (CISC) instruction set, such as the x86 instruction set, or from a reduced instruction set computer (RISC) instruction set, such as the PowerPC™ instruction set.

Two instruction decode units are provided so that instructions from either instruction set may be decoded. Two instruction decoders are required when the instruction sets are separate because the instruction sets each have an independent encoding of operations to opcodes. For example, both instruction sets have an ADD operation or instruction. However, the binary opcode number which encodes the ADD operation is different for the two instruction sets. In fact, the size and location of the opcode field in the instruction word is also different for the two instruction sets. In the x86 CISC instruction set, the opcode 03 hex is the ADD r,v operation or instruction for a long operand. This same opcode, 03 hex, corresponds to a completely different instruction in the PowerPC™ RISC instruction set. In CISC the 03 hex opcode is an addition operation, while in RISC the 03 hex opcode is TWI--trap word mediate, a control transfer instruction. Thus two separate decode blocks are necessary for the two separate instruction sets.

Programs may run in either or both instruction sets. Data and other information may be shared between RISC programs and CISC programs. One way to share data and other information is to store the data in a register within the CPU before switching to the alternate instruction set, and making registers readable by either instruction set. Unfortunately, this requires that the instruction sets be extended to provide instructions to read the additional registers. The shared data could also be saved to a stack in memory, but this decreases performance due to the time required to transfer the data to memory and to adjust the stack pointers.

Two sets of registers could be provided; one set for the use of CISC programs and a second set for the use of RISC programs. This is an expensive approach since the registers reside on the CPU die, which has a limited space available for registers. The additional registers require increasing the size of the CPU die, or deleting another function such as floating point processing.

Floating point registers should also be shared. Floating-point numbers include a fraction or mantissa portion and an exponent portion. Wider formats for floating point data are typically used than for integer data. Often 64 or 80-bit formats are used for floating point numbers, while a 32-bit format is used for integers. Also, CISC may support a wider floating point format of 80 bits while RISC only provides for a 64-bit format. A 128-bit extended format is known in the art for both RISC and CISC. Separate status flags and mask or enable bits are provided for floating point operations.

What is desired is a way to share some of the registers between a CISC and a RISC architecture on a dual-instruction-set CPU. It is further desired to have shared registers for data and system information. The shared registers should not be extra registers in addition to the registers already defined by the CISC or RISC architectures, but should be registers already existing in the architectures. The shared registers must not cause conflicts between use in the two instruction sets or other undesirable effects. It is also desired to share floating point data, status, and control registers between the two architectures.

SUMMARY OF THE INVENTION

Certain CPU registers defined by a RISC and a CISC architecture are shared. CISC and RISC programs may alter and read these shared registers, allowing data and system information to be exchanged between programs running in the two instruction sets.

A shared register system for a dual-instruction-set processor has a shared register for storing information to be transferred between a first program comprised of instructions from a first instruction set and a second program comprised of instructions from a second instruction set. The first instruction set has a first encoding of operations to opcodes, while the second instruction set has a second encoding of operations to opcodes. The first encoding of operations to opcodes is substantially independent from the second encoding of operations to opcodes.

A first means is for accessing the shared register from the first instruction set. The first means writes information into the shared register responsive to a first subset of instructions from the first instruction set. A second means is for accessing the shared register from the second instruction set. The second means reads information from the shared register responsive to a second subset of instructions from the second instruction set.

The invention allows information to be transferred from the first program to the second program using the shared register. In other aspects of the invention, the shared register may be any one of the general-purpose registers accessible to both instruction sets, while the source and destination fields in the instruction words specify which general-purpose register to access. In still further aspects of the invention, the shared register is the flags register which stores flags or condition codes that are implicitly written by arithmetic-logic-unit (ALU) operations. Although the shared flags register contains a first flags field for flags from the first instruction set and a second flags field for the flags from the second instruction set, either instruction set can access the flags in the shared register regardless of which instruction set the flags are from.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a RISC register set.

FIG. 2 is a diagram of a CISC register set.

FIG. 3 is a diagram of a CISC condition flag register and a RISC condition register.

FIG. 4 is a diagram showing that the RISC floating point status and control register (FPSCR) and the CISC floating point status register and CISC floating point control register may be combined.

FIG. 5 shows in detail how RISC floating point status and control register (FPSCR) and the CISC floating point status register and CISC floating point control register are combined.

FIG. 6 is an implementation of RISC floating point data register pairing for CISC extended precision.

FIG. 7 shows floating point data register file having two write and four read ports.

FIG. 8 shows shared registers in a dual-instruction-set CPU.

DETAILED DESCRIPTION

The present invention relates to an improvement in floating point processor architecture. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.

This application is related to the copending application for a "Dual-Instruction-Set Architecture CPU with Hidden Software Emulation Mode", filed Jan. 11, 1994, U.S. Ser. No. 08/179,926 hereby incorporated by reference.

A dual-architecture central processing unit (CPU) is capable of operating in three modes--RISC mode, CISC mode, and emulation mode. A first instruction decoder decodes instructions when the processor is in RISC mode, while a second instruction decoder decodes instructions while the processor is in CISC mode. Two instruction decoders are needed since the RISC and CISC instruction sets have an independent encoding of instructions or operations to binary opcodes.

The third mode of operation, emulation mode, also uses the first instruction decoder for RISC instructions, but emulation mode executes a superset of the RISC instruction set. Using emulation mode, individual CISC instructions may be emulated with RISC instructions. Thus, not all CISC instructions need to be directly supported in the CPU's hardware. Unsupported CISC instructions cause a jump to an emulation mode routine to emulate the unsupported CISC instruction. Upon completion of the emulation mode routine, control is returned to the CISC program with the next CISC instruction.

RISC INSTRUCTIONS NEED ACCESS TO CISC REGISTERS

Emulation of CISC instructions with RISC instructions creates a need for the RISC instructions to have access to CISC registers. For example, a CISC branch instruction may be emulated by an emulation routine of RISC instructions. The CISC branch instruction may be a conditional branch that only branches if a certain bit in a condition code register is set, perhaps by a previous CISC instruction. Since the CISC condition code register is part of the CISC architecture, but not the RISC architecture, the condition code register is not visible to the RISC instructions in the emulation routine. However, the RISC emulation routine must have access to this CISC condition code register to determine if the branch should be taken.

The more complex CISC floating point instructions may also be emulated by a routine of RISC floating point instructions. These RISC floating point instructions thus have to be able to access the CISC floating point data registers. The CISC control and status registers must also be accessed by the RISC floating point instructions to determine if an exception has been enabled, and to set the status flags depending upon the result of the RISC floating point operations and the CISC enable bits. For example, the CISC enable bits may enable exception reporting for divide-by-zero, but not for overflow.

The RISC instructions emulating the complex CISC instruction must first read the divide-by-zero and overflow enable bits in the CISC control register, and then set the divide-by-zero or overflow flags in the CISC status register if a divide-by-zero or overflow exception occurs when the RISC instructions are executed when the divide-by-zero or overflow bit is enabled in the CISC control register.

RISC REGISTER SET

FIG. 1 is a diagram of a register set for a RISC architecture such as the PowerPC™. Registers that are visible to a user program are shown as user register space 10. Supervisory programs such as operating systems are able to see all of the registers in the user register space 10 and the registers in the supervisor's register space 12. The user registers include general-purpose registers 14 which are used by programs for temporary storage of operands and results, and for address formation. Floating point data registers 16 are provided for storing floating point numbers that a numeric processor operates on. Condition register 20 contains condition codes set by various instructions and is useful for setting and checking conditions for conditional branch instructions. Integer exception register 18 contains bits that are versions of overflow bits. It contains information on overflows and carries that occurred in an arithmetic-logic-unit (ALU) when the instruction was executed. Link register 22 contains the branch target address when a special branch to link register instruction is executed. Count register 24 holds a value for a loop count which can be decremented, providing a simple way of programming loops.

A supervisory program such as an operating system has access to additional registers in the supervisor's register space 12. Supervisor general-purpose registers 26 are for general use by the supervisory program. Segment registers 28 and block-address translation registers 32 are for address translation functions. Machine state register 36 defines the state of the processor, including reset, and CISC/RISC/emulation mode. Machine state register 36 contains a privilege-level bit, the PR bit, to indicate if the processor is running in user or supervisor mode when RISC mode is active. An additional bit, the xE bit, is included in machine state register 36 to indicate CISC and emulation modes. The xE bit and the PR bit are encoded as shown in Table 1.

                  TABLE 1                                                          ______________________________________                                         Machine Status                                                                 xE bit        PR bit  Processor Mode                                           ______________________________________                                         0             0       RISC Supervisor                                          0             1       RISC User                                                1             0       x86 Emulation                                            1             1       x86 CISC                                                 ______________________________________                                    

Machine status save restore 0 register 30 saves the effective address of the instruction following the instruction causing an exception or a system call instruction. Machine status save restore 1 register 34 saves part of machine state register 36 and other information on the cause of an exception when an exception occurs. In particular, machine status save restore 1 register 34 receives the PR bit and xE bit from the machine state register 36. Machine status save restore 1 register 34 thus saves the mode the processor was in at the time of an exception or other event, and is used to restore the processor to that mode when exception processing is complete. A return from interrupt (rfi) instruction at the completion of the exception processing restores the xE and PR bits to the machine status register 36 from machine status save restore 1 register 34. Together, machine status save restore registers 30, 34 save the state of the processor when an exception occurs, allowing the system to return to the user program once the exception handler routine is completed. Hardware-specific registers 38 contain miscellaneous implementation-specific information such as extended features.

Floating-point status and control FPSCR register 52 contains control or enable bits which enable reporting of different types of exceptions that can occur during floating point operations. FPSCR register 52 also contains flags or status bits that are set by the floating point operation, such as zero or negative result flags and overflow flags.

CISC REGISTER SET

FIG. 2 is a diagram of a register set for a CISC architecture such as the x86 used in microprocessors by Intel corporation of Santa Clara, Calif., Advanced Micro Devices of Sunnyvale, Calif., and Cyrix Corporation of Richardson, Tex. Registers that are visible to a user program are shown as user register space 11. Supervisory or system-level programs such as operating systems are able to see all of the registers in the user register space 11 and the registers in the system-level register space 13. The user registers include general-purpose registers 15 which are used by programs for temporary storage of operands and results, and for address formation. Segment registers 17 are provided for generating linear addresses. Floating point data registers 50 store data in a floating-point format, including the fraction or mantissa portion and the exponent and sign. Flags register 21 contains flags or condition codes set by various instructions and is used for setting and checking conditions for conditional branch instructions. Instruction pointer 19 contains the address of the instruction currently being executed.

Floating-point control register 55 contains control or enable bits which enable reporting of different types of exceptions that can occur during floating point operations. Floating point status register 57 contains flags or status bits that are set by the floating point operation, such as zero or negative result flags and overflow flags.

A system-level program such as an operating system has access to additional registers in the system-level register space 13. Control registers 31, 35, 39 define the state of the processor, including protected or real modes, exception handling, cache enabling, and contain the base address of page tables. Breakpoints and control, performance monitoring, real-time clocks and other control may also be included in control registers 31, 35, 39. Floating point instruction pointer register 38 contains the address of the instruction last executed by the floating point unit. It must be saved because a floating point exception may be signaled after other instructions pass through the integer pipeline. It allows an exception handler routine to find the exact address causing a floating point exception.

CONDITION CODE/FLAGS REGISTER

Condition codes are employed by both RISC and CISC instruction sets. Instructions that use the arithmetic-logic-unit (ALU) or floating point unit (FPU) may produce a result having a zero or negative value. These instructions cause a flag or condition code in register 21 or 20 to be set when the result is zero or negative. Floating-point instructions may also set a zero flag in status register 57 or FPSCR register 52. Iterative loops may be programmed using such flags. For example, a simple loop may execute a series of instructions and decrement a loop variable each time the series of instructions in the loop is executed. The loop variable is initially set to the number of times to execute the loop. At the end of the loop, an ALU instruction subtracts one from the loop variable. When the loop variable becomes zero, the zero flag is set. A conditional branch instruction checks the zero flag and exits the loop when zero is reached.

Many other flags may be defined. For example, the x86 CISC EFLAGS register defines the flags of Table 2 that are set or cleared by ALU instructions depending on the result of the instruction.

                  TABLE 2                                                          ______________________________________                                         CF    Carry Flag     Set if carry-out or borrow                                PF    Parity Flag    Set if low 8 bits have even parity                        AF    Auxiliary (2nd) Carry                                                                         Bit 3 carry-out, used for BCD                             ZF    Zero Flag      Set if all bits are zero                                  SF    Sign Flag      Set if highest-order bit is one                           DF    Direction Flag Incr. or Decr. Addr. (String Instr)                       OF    Overflow Flag  Signed overflow to highest bit                            ______________________________________                                    

Other bits in the x86 EFLAGS register are not flag bits set by operations but are control bits that define how the processor operates. Table 3 shows these control bits.

                  TABLE 3                                                          ______________________________________                                         TF    Trap Flag        Trap after next instruction                             IF    Interrupt enabled Flag                                                                          Enables external interrupts                             IOPL  Input/Output Privilege level                                                                    Max. Privilege for I/O instr.                                 (2 bits)                                                                 NT    Nested Task Flag Nested task being executed                              RF    Resume Flag      Resume after breakpoint                                 VM    Virtual Mode     Virtual 8086 mode executing                             AC    Memory Alignment Check                                                                          Mis-aligned data will fault                             ______________________________________                                    

The dual-instruction-set processor directly executes only the simpler CISC instructions. Many of these simpler CISC instructions set or clear the flag bits in Table 2. However, the control bits in Table 3 are set or cleared by complex or infrequently used CISC instructions such as privileged instructions. These instructions are therefore emulated. Only the simple CISC instructions modify the flag bits in the CISC EFLAGS register. Emulated instructions modify the control bits in the CISC EFLAGS register.

FIG. 3 shows the CISC flags register 21 and the RISC Condition register 20. In the PowerPC™ RISC architecture, flags or condition codes are kept in the condition register (CR) 20. Condition register 20 is a 32-bit register divided into eight 4-bit fields, CR0 to CR7. Most RISC integer instructions generate the four bits in CR0, but do not modify any bits in fields CR1 to CR7. Table 4 shows the meanings of the four bits in CR0.

                  TABLE 4                                                          ______________________________________                                         RISC CR0 field                                                                 CR0 bit    Name       Description                                              ______________________________________                                         0          Negative   Result is negative                                       1          Positive   Result is positive and not zero                          2          Zero       Result is Zero                                           3          Overflow   Overflow has occurred                                    ______________________________________                                    

Floating point operations do not modify the bits in the CR0 field, but they do modify the four bits in the CR1 field. Table 5 shows the definitions of the four bits in CR1 set by floating point operations.

                  TABLE 5                                                          ______________________________________                                         RISC CR1 field                                                                 CR1 bit                                                                              Name          Description                                                ______________________________________                                         0     FP exception  Floating point exception has occurred                      1     FP enabled exception                                                                         A floating point enabled exception has                                         occurred                                                   2     FP invalid exception                                                                         An invalid floating point exception has                                        occurred                                                   3     FP Overflow   Floating Point Overflow has occurred                       ______________________________________                                    

A RISC compare instruction can set bits in any of the 4-bit fields CR0-CR7. Table 6 shows the definitions for the four bits in any field CRn set by the RISC compare instruction.

                  TABLE 6                                                          ______________________________________                                         RISC CRn field set by Compare Instruction                                      CRn bit                                                                              Name      Description                                                    ______________________________________                                         0     Less Than register A is less than register B or immediate                                value from instruction word                                    1     Greater Than                                                                             register A is greater than register B or                                       immediate value from instruction word                          2     Equal To  register A is equal to register B or immediate                                 value from instruction word                                    3     Overflow  Copy of the Overflow bit in XER register                       ______________________________________                                    

The compare instruction specifies which field to write its result to. Likewise, the RISC branch instructions can specify, as a condition for branching, any bit in any of the fields CR0 to CR7. Thus the programmer may write condition codes to the other six fields in the CR register 20. The programmer may later use these other fields with the branch instruction using any of the bits in any of the fields CR0 to CR7. RISC move instructions may also load bits into any of the fields CR0 to CR7 of the CR register 20. The RISC move instruction may move bits from another register, or from one 4-bit CR field to another field within CR register 20. A mask may be specified in the move instruction word to indicate which bits to move and which bits to not modify. A wealth of RISC logical instructions are provided that specify as inputs one or two bits in any of the 4-bit fields. A Boolean logical function is performed on the specified input bits, and the resulting output bit is written to any bit in any of the 4-bit fields in CR register 20. Thus RISC provides a variety of instructions to update, modify, and perform logical functions on parts of the CR register 20.

CISC AND RISC FLAGS REGISTERS MAY BE COMBINED

Although fields CR2 to CR7 may be updated in a variety of ways by RISC instructions, the inventors have recognized that fields CR2 to CR7 may be infrequently updated while fields CR0 and CR1 are typically updated frequently. A programmer has to explicitly decide to update fields CR2 to CR7, while fields CR0 and CR1 are implicitly updated by many RISC instructions.

The inventors have also recognized that most CISC instructions update bits 0 to 11 in the CISC EFLAGS register, while few CISC instructions Update bits 12 to 31 in the CISC EFLAGS register. FIG. 3 compares the CISC flags register 21 and the RISC Condition register (CR) 20. FIG. 3 shows that simple CISC integer instructions update flags in bit-positions 0 to 11 of CISC EFLAGS register 31, while RISC integer instructions update bits in the 4-bit CR0 field. RISC floating point instructions update field CR1, while few RISC instructions update fields CR2 to CR7. If a RISC programmer can avoid using fields CR5 to CR7 in RISC CR register 20, which correspond to bit-positions 11 to 0 in the CISC EFLAGS register 21, then the CISC EFLAGS register 21 can be folded into or combined with the RISC CR register 20. Since the RISC programs that share data with CISC programs are typically RISC emulation routines, the RISC programmer is aware of these limitations and can avoid using fields CR5 to CR7. Standard RISC user programs that do not avoid using fields CR5 to CR7 are not able to take advantage of the data sharing features of the invention, but they are still able to take advantage of the cost savings of the invention because fewer registers are needed on the microprocessor. However, x86 CISC emulation routines written in RISC code greatly benefit by both sharing data using the shared registers and by cost savings.

Complex CISC instructions modify the control bits of Table 3, which are in bit-positions 12 to 21 of EFLAGS register 21. These complex CISC instructions may be emulated with RISC instructions in the emulation mode of the dual-instruction-set processor. These control bits may be stored in memory rather than in EFLAGS register 21, freeing up these bits for use by CR2 to CR4.

EMULATION OF CISC ENHANCED BY COMBINED FLAGS REGISTER

The CISC EFLAGS register 21 and the RISC CR register 20 are combined into a single 32-bit register in the dual-instruction-set processor. When a complex CISC instruction that updates a control bit in the EFLAGS register 21 is emulated, the RISC instructions in the emulation routine merely have to update the corresponding bit in one of the fields CR2 to CR4 in the RISC CR register 20, because the RISC CR register 20 and the CISC EFLAGS register 21 are the same shared register. For example, a complex CISC instruction ASCII Adjust for Add (AAA) performs an add operation with an adjustment or conversion for a decimal format. Sometimes a carry is generated by the AAA instruction, which then writes a one to the Carry Flag bit (CF) at bit-position 0 in the EFLAGS register 21. This complex CISC AAA instruction is not supported by the instruction decoder and signals an unsupported opcode exception, which causes emulation mode to be entered from CISC mode. An emulation routine of RISC instructions is executed to emulate the complex CISC instruction. This emulation routine contains separate add and ASCII conversion instructions and a RISC Boolean instruction which sets bit 3 in CR7, corresponding to bit-position 0, the CF bit, in the EFLAGS register 21. The RISC Boolean instruction setting this bit may be a CR-register Boolean XNOR instruction (creqv) that exclusive-NOR's bit 3 to itself, with field CR7 as its sources and destination. Once the emulation routine is completed, CISC mode is again entered and execution of the CISC program resumes at the following CISC instruction. This following CISC instruction may be an add-with-carry (ADC) instruction which is not emulated but directly reads the carry flag CF set by the emulated AAA instruction. Because CISC EFLAGS register 21 and RISC CR register 20 are implemented as the same hardware register on the CPU die, updating the RISC register also updates the register seen by CISC programs.

The emulation routine of RISC instructions, or other native RISC programs, may freely update bits in fields CR0 and CR1, because these bits correspond to reserved bits in CISC EFLAGS register 21. At the conclusion of the emulation routine, before CISC mode is entered, these bits in CR0 and CR1 are cleared so that they are all read as zero when CISC mode instructions read CISC EFLAGS register 21.

BENEFITS AND USES OF MERGED FLAGS REGISTERS

Folding CISC EFLAGS register 21 and RISC CR register 20 together brings additional benefits besides cost reduction by having fewer registers on the CPU die. RISC programs can examine the flag bits in CISC EFLAGS register 21 to determine the results generated by the CISC program using the existing RISC instructions. No special RISC instructions are needed to examine this information from the CISC program. The RISC program may examine the zero flag to determine if the CISC program had a zero result, which might indicate the end of an iterative loop. The CISC zero flag (ZF) at bit position 6 may be examined by a RISC instruction simply by reading bit 1 of field CR6. Likewise, any of the other flag bits may be examined by a RISC program by reading the corresponding bit in the RISC CR register 20. Particularly with emulation routines, having this information is critical. Because the RISC instruction set has so many instructions which can access RISC CR register 20 directly, the emulation routine may be efficiently programmed without many move or load/store instructions to make available the CISC EFLAGS register 21. Thus the emulation routine has a much higher performance than if the CISC EFLAGS register 21 had to be stored on a stack in memory and retrieved for the emulation routine to examine. Even moving a separate CISC EFLAGS register 21 from one CPU register into the RISC CR register 20 for use by RISC branch instructions require extra RISC instructions, decreasing performance relative to the invention.

The emulation routine can perform branches directly off the CISC flag bits. A RISC instruction in the emulation routine can branch off the CF or AF bits in the CISC EFLAGS register by merely specifying the corresponding bit in the RISC CR register. The CR register is the most visible and accessible state register in the PowerPC™ RISC architecture. The EFLAGS register in the x86 CISC architecture is likewise the most interesting CISC register because of the many state flags stored in it. Using the RISC CR register as a window into the CISC architecture provides a versatile and powerful tool.

The RISC and CISC condition code and flags registers are effectively merged together into a single flags register that is accessible by instructions from both instruction sets. The merged register is special because it is not just explicitly accessible as a register, but the merged register is also accessible implicitly. Instructions that implicitly update the flags register, whether RISC or CISC instructions, update the same merged register. Because the two instruction sets tend to use separate portions of the merged register, each instruction set can use its portion of the merged register, without interfering with the other instruction set. Yet programs running in one instruction set can still observe the flags set by programs in the other instruction set. Thus information about the results generated by one instruction set may be made available to programs in the other instruction set.

Information about the operating state of the x86 CISC program is also available by reading the merged flags register since control bits are stored in bit-positions 11 to 21. These control bits, shown in Table 3, include virtual 8086 mode, interrupt enabling and privilege levels, an indication of task nesting, debug trapping, and data alignment checking. Again, a RISC program merely has to read the proper bit in the merged register, which appears as the standard RISC CR register to the RISC program. Often the register does not even have to be explicitly read by the RISC program, but only implicitly read. A RISC conditional branch instruction can be set to branch on the bit in CR 5 corresponding to the CISC interrupt enable control bit (IF). A RISC program could branch to a routine to check and disable interrupts if the IF bit is set, but continue without disabling interrupts if the bit is zero, knowing that interrupts are not possible. Thus the CISC interrupt enable bit is used to direct program flow in the RISC program merely by branching on the shared CISC/RISC bit. No register transfers, loads, or even explicit reads were necessary. The invention provides a very clean, simple, and efficient way to pass information between programs running in two different instruction sets.

FLOATING POINT REGISTERS COMBINED

FIG. 4 is a diagram showing that the RISC floating point status and control register FPSCR 52 and the CISC floating point status register 57 and CISC floating point control register 55 may be combined. Both the RISC FPSCR register 52 and CISC floating point status register 57 contain status or flag bits which indicate when a floating point exception has occurred or when the result is zero, negative, etc. Both registers 52, 57 also contain one or more summary bits that indicate if any exception has occurred. This allows system software to read just the summary bit to determine if an exception has occurred.

Both the RISC FPSCR register 52 and CISC floating point control register 55 contain control or enable bits which enable the reporting of various kinds of floating point exceptions. Control of rounding may also be controlled by rounding control bits in these registers.

Since both the CISC and RISC floating point status and control registers perform similar functions, the inventors have realized that these registers may be combined into a single register accessible by both instruction sets.

FIG. 5 shows in detail how RISC floating point status and control register FPSCR 52 and the CISC floating point status register 57 and CISC floating point control register 55 are combined. Tables 7, 8, 9 show the bits in RISC FPSCR register 52, and the corresponding bits in CISC floating point status register 57 and CISC floating point control register 55. While the order of the bits in the CISC registers is not identical to the order of the corresponding bits in RISC FPSCR register 52, the CISC bits may nevertheless be mapped into RISC FPSCR register 52. When a CISC instruction reads or writes to the CISC registers 55, 57, the bits from RISC FPSCR register 52 must be re-ordered to the CISC order, either by a simple hardware mapping or by emulation software.

                  TABLE 7                                                          ______________________________________                                         CISC FP Status Register                                                        Bit  Symbol  Name        Meaning                                               ______________________________________                                         15   B       Error Summary                                                                              Equal to ES bit 7                                     14   C3      Condition Code                                                                             Similar to EFLAGS condition codes                     13:11                                                                               TOP     Stack Top   Points to top of FP data stack                        10:8 C2:C0   Condition Codes                                                                            Similar to EFLAGS condition codes                     7    ES      Error Summary                                                                              Set if any exceptions detected, even                                           if not enabled                                        6    SF      Stack Flag  Stack overflow or underflow                           5    PE      Precision Flag                                                                             Not precise result; rounding occurred                 4    UE      Underflow Flag                                                                             Result is too small to be represented                 3    OE      Overflow Flag                                                                              Result is too large to be represented                 2    ZE      Zero-Divide Flag                                                                           Divisor was zero                                      1    DE      Denormalized                                                                               An operand is denormalized (operand                                Flag        has smallest exponent but non-zero                                             mantissa)                                             0    IE      Invalid Op Flag                                                                            Operation on a NaN, infinity,                                                  unsupported format, etc.                              ______________________________________                                    

                  TABLE 8                                                          ______________________________________                                         CISC FP Control Register                                                       Bit  Symbol  Name        Meaning                                               ______________________________________                                         11:10                                                                               RC      Round Control                                                                              Rounding type and direction                           9:8  PC      Precision Control                                                                          Size of mantissa used                                 5    PM      Precision Mask                                                                             Enable mask for Precision                                                      Exception                                             4    UM      Underflow Mask                                                                             Enable mask for Underflow                                                      Exception                                             3    OM      Overflow Mask                                                                              Enable mask for Overflow Exception                    2    ZM      Zero-Divide Enable mask for Zero-Divide                                        Mask        Exception                                             1    DM      Denormalized                                                                               Enable mask for Denormalized                                       Mask        Exception                                             0    IM      Invalid Op Mask                                                                            Enable mask for Invalid Op                                                     Exception                                             ______________________________________                                    

                  TABLE 9                                                          ______________________________________                                         FPSCR Register for RISC and CISC                                                    RISC     CISC                                                             Bit  Symbol   Symbol  Name     Meaning                                         ______________________________________                                         0    FX       ES      Error Summary                                                                           Set if any exceptions                                                          detected, even if not                                                          enabled                                         1    FEX              Enabled Error                                                                           Set if any enabled                                                    Summary  exception is detected                           2    VX       IE      Invalid Op Flag                                                                         Invalid Operation                               3    OX       OE      Overflow Flag                                                                           Result is too large to be                                                      represented                                     4    UX       UE      Underflow Flag                                                                          Result is too small to be                                                      represented                                     5    ZX       ZE      Zero-Divide                                                                             Divisor was zero                                                      Flag                                                     6    XX       PE      Precision Flag                                                                          Loss of precision; rounding                                                    occurred                                        7    VXSNAN           NaN Flag NaN exception detected                          8    VXISI            Subtract Exception detected by                                                 Infinity Flag                                                                           subtracting infinity                            9    VXIDI            Divide Infinity                                                                         Exception detected by                                                 Flag     dividing infinity                               10   VXZDZ            Divide-by-Zero                                                                          Exception detected by                                                 Flag     dividing by zero                                11   VXIMZ            Multiply Exception detected by                                                 Infinity ×                                                                        multiplying infinity by zero                                          Zero Flag                                                12   VXVC     *DE     NaN Compare                                                                             Exception detected by                                                 Flag     compare of NaN                                  13   FR       *PC     Incremented                                                                             Incremented intermediate                                              Flag     result                                          14   FI       *PC     Round Flag                                                                              Rounded intermediate result                     15   C                Result Codes                                                                            Indicates type of number                                                       returned                                        16   FL       C0      Result Codes                                                                            Indicates type of number                                                       returned                                        17   FG       C1      Result Codes                                                                            Indicates type of number                                                       returned                                        18   FE       C3      Result Codes                                                                            Indicates type of number                                                       returned                                        19   FU       C2      Result Codes                                                                            Indicates type of number                                                       returned                                        20                    Reserved                                                 21   VXSOFT           S/W Exception                                                                           S/W Requested Exception                                               Flag                                                     22   VXSQRT           SQRT Flag                                                                               Invalid square-root                                                            exception detected                              23   VXCVI    *DM     Integer NaN                                                                             Integer convert of NaN                                                Flag     exception detected                              24   VE       IM      Invalid Op                                                                              Enable mask for Invalid Op                                            Mask     Exception                                       25   OE       OM      Overflow Enable mask for Overflow                                              Mask     Exception                                       26   UE       UM      Underflow                                                                               Enable mask for Underflow                                             Mask     Exception                                       27   ZE       ZM      Zero-Divide                                                                             Enable mask for Zero-                                                 Mask     Divide Exception                                28   XE       PM      Precision Mask                                                                          Enable mask for Precision                                                      Exception                                       28   XE       PM      Precision Mask                                                                          Enable mask for Precision                                                      Exception                                       29   NI       NI      IEEE Mode Bit                                                                           1=CISC FP Mode, 0=RISC                          30:31                                                                               RN       RC      Round Control                                                                           Rounding type and                                                              direction                                       ______________________________________                                    

While most status and control bits overlap for the two instruction sets, a few CISC bits have no corresponding RISC bit. Thus some of the RISC bits in the FPSCR register have an alternate, unrelated function for CISC mode. These CISC bits are PC, DE, and DM, which are marked in Table 9 with asterisks to indicate that the RISC functionality shown does not apply for CISC mode. For example, bit 23 is VXCVI in RISC mode, which is the integer NaN flag, which is set when a conversion of NaN (not a valid number) to an integer is attempted. However in CISC mode, bit 23 is DM, Denormalized Mask which enables reporting of Denormalized Exceptions.

The CISC precision control PC, bits 13, 14, are shared with the RISC FR, FI bits which indicate the type of rounding last performed. In CISC mode these bits 13, 14 are control bits rather than status bits, controlling the precision (either 24, 53, or 64-bit mantissas).

For the rounding control, bits 30, 31, the RISC encoding is used for both CISC and RISC mode:

00=Round to even if tie, else round to nearest.

01=Round to zero.

10=Round to positive infinity.

11=Round to negative infinity.

When the IEEE Mode Bit is set, very small numbers that are smaller than the smallest normalized number are rounded to zero. However, when the IEEE bit is cleared, these very small numbers are not rounded to zero but are left as denormal numbers. Other bits in the CISC floating point status and control registers are contained in registers in the instruction decode unit. For example, the stack-top pointer, TOP, and the stack fault bits SF are not included in the FPSCR register.

Thus the two CISC floating point status and control registers may be folded into the single RISC floating point status and control register FPSCR. This allows RISC floating point instructions to read the status information from CISC floating point instructions without having to physically move the CISC status bits from a CISC floating point status register to another register.

ADDRESS GENERATION REGISTERS COMBINED

Other registers may also be folded together or combined. The RISC count register (CTR) is a 32-bit register that contains a loop count that can be decremented when a branch instruction is executed. It can be explicitly accessed by some RISC move instructions, and can be implicitly accessed by certain RISC branch instruction which cause the CTR register to be read and decremented.

One of the CISC segment registers (17 of FIG. 2) holds the base address of the code segment. This code segment register is needed to generate addresses for fetching instructions when in CISC mode. The base address in the code segment register is also needed to calculate the targets of a branch instruction. Thus the code segment register is accessed frequently.

The RISC CTR register and the CISC code segment registers may be combined together in the dual-instruction-set processor. The combined register holds the CISC code segment base address during CISC mode. The code segment base address is left in the combined CTR/CS register when RISC mode is entered. The code segment base address is restored by the emulation routine to the combined CTR/CS register before CISC mode is re-entered.

Since the CTR register is infrequently used, the code segment can remain in the combined CTR/CS register most of the time, even during RISC mode. RISC emulation routines may be programmed that do not use the CTR register, thus increasing performance of the emulation routine. Since both the CISC code segment register and the RISC count register are needed by the branching unit, merging these into the same register provides a single shared register to supply both CISC and RISC address information to the branching unit.

RETURN ADDRESS REGISTERS COMBINED

The RISC machine status save restore 0 register (SRR0) 30 of FIG. 1 saves the effective address of the instruction following the instruction causing an exception, or the effective address of the instruction following a system call instruction. When the exception handler routine completes and a return-from-interrupt (rfi) instruction is executed, the address that was stored in SRR0 is reloaded into the instruction pointer so that program execution can continue with the next instruction. Thus the SRR0 register provides a place for the address of a RISC instruction that occurs after a RISC instruction causing an exception.

In the dual-instruction-set processor, when a CISC program causes an exception, a RISC emulation routine is called and executed. Thus the normal CISC exception handling hardware is not needed and no counterpart to the SRR0 register is necessary. The CISC CR2 register normally holds the address of an instruction causing a page fault in a CISC-only processor. This CR2 register is not needed in the dual-instruction-set processor since page faults, like other exceptions, are all handled by RISC emulation code. Other CISC exceptions cause the instruction pointer (IP) to be pushed on the stack in memory by the exception handier. Pushing the IP to the stack is not performed in micro-code, as in prior-art x86 processors, but by the emulation routine for the dual-instruction-set processor. Any time emulation mode is entered from CISC mode, regardless of the cause, the address of the CISC instruction being executed is stored into the RISC SRR0 register. When the emulation routine completes, the processor switches back to CISC mode, the address stored in register SRR0 is re-loaded into the instruction pointer, and the next instruction in the CISC program is fetched and executed.

If emulation mode is entered to handle an exception, then SRR0 should point to the CISC instruction causing the exception, so that CISC instruction can be re-started once the exception handling is complete. When the emulation routine determines the size of the CISC instruction, the size is added to the address stored in the SRR0 register to get the address of the next CISC instruction. This addition must occur because CISC instructions can vary in size, CISC instructions being 1-15 bytes in size. Thus SRR0 causes the CISC program to continue at the instruction following the CISC instruction being emulated, unless an exception occurs.

The RISC SRR0 register, which normally holds the address of an instruction causing an exception, is also used to hold the address of a CISC instruction that caused emulation mode to be entered because the CISC instruction was not supported in hardware but had to be emulated. These are two parallel uses, but for two different instruction sets. The additional hardware to support both of these functions is minimal because these functions are closely related.

LR AND FP REGISTERS COMBINED

The RISC link register (LR) provides a branch target address for a RISC branch conditional to link instruction. It is a 32-bit register. While most RISC branch instructions do not use the link register, some do, such as the RISC branch-and-link instruction.

The CISC architecture requires that the address of a floating point instruction be saved. Since floating point operations may take several clock cycles to complete, several simple integer instructions could have completed execution by the time an exception is signaled that was caused by the floating point instruction. Storing the address of the floating point instruction allows the exception handling routine to backtrack the code and sort out the integer instructions executed. This address of the floating point instruction is stored in the FP IP register in the CISC architecture. In the dual-instruction-set processor this address is instead stored in the RISC link register.

Storing the CISC floating point instruction's address in the RISC link register may cause a problem if a RISC program contains a RISC instruction that uses the link register. If that happens, the CISC floating point instruction's address must be saved to a stack in memory or to another general-purpose register. It is believed that this is an infrequent occurrence and therefore the cost savings of combining the registers justifies sharing these two registers. Most of the time the RISC link register is not used, so no conflict occurs.

Since the link register is used to store the address of the RISC target, a path for an instruction address is already provided to this register. Thus the floating point instruction's address may also use this instruction path to the shared LR register.

GPR'S COMBINED

There are 32 general-purpose registers (GPR's) for RISC which may be explicitly read or written by integer RISC instructions. The x86 CISC architecture provides only 8 general-purpose registers which can be read or written by a user program. CISC also provides 6 segment registers which contain segment base addresses that are used to calculate the linear address of code, data operands, and a stack. These segment registers can only be used for segmentation, a part of address generation, and have restrictions on reading and writing them with CISC instructions. Only x86 privileged or segment-load instructions can read or write the segment registers. All of these instructions are emulated by the dual-instruction-set processor. Thus CISC-mode instructions cannot directly read or write these segment registers.

These 32 RISC general-purpose registers may be merged with the 8 CISC general-purpose registers and the 6 CISC segment registers. Table 10 shows how these registers are used for RISC, CISC, and emulation mode.

                  TABLE 10                                                         ______________________________________                                         Shared GPR's                                                                   RISC Mode   CISC Mode  Emulation Mode                                          ______________________________________                                         GPR 0       EAX GPR    EAX GPR                                                 GPR 1       ECX GPR    ECX GPR                                                 GPR 2       EDX GPR    EDX GPR                                                 GPR 3       EBX GPR    EBX GPR                                                 GPR 4       ESP GPR    ESP GPR                                                 GPR 5       EBP GPR    EBP GPR                                                 GPR 6       ESI GPR    ESI GPR                                                 GPR 7       EDI GPR    EDI GPR                                                 GPR 8       ES Seg Base                                                                               ES Seg Base                                             GPR 9       CS Seg Base                                                                               CS Seg Base                                             GPR 10      SS Seg Base                                                                               SS Seg Base                                             GPR 11      DS Seg Base                                                                               DS Seg Base                                             GPR 12      FS Seg Base                                                                               PS Seg Base                                             GPR 13      GS Seg Base                                                                               GS Seg Base                                             GPR 14      N/A        Emulation Base Address                                  GPR 15      N/A        0 Base Address                                          GPR 16      N/A        GPR 16                                                  GPR 17      N/A        GPR 17                                                  GPR 18      N/A        GPR 18                                                  GPR 19      N/A        GPR 19                                                  GPR 20      N/A        GPR 20                                                  GPR 21      N/A        GPR 21                                                  GPR 22      N/A        GPR 22                                                  GPR 23      N/A        GPR 23                                                  GPR 24      N/A        GPR 24                                                  GPR 25      N/A        GPR 25                                                  GPR 26      N/A        GPR 26                                                  GPR 27      N/A        GPR 27                                                  GPR 28      N/A        GPR 28                                                  GPR 29      N/A        GPR 29                                                  GPR 30      N/A        Emulation Assist Address                                GPR 31      N/A        Emulation Assist Data                                   ______________________________________                                    

Table 10 shows that in RISC mode, the 32 general-purpose registers are accessible as true general-purpose registers. Any of the 32 registers may be read or written by RISC programs. In CISC mode, there are only 8 general-purpose registers, EAX through EDI, which share the same physical registers with the RISC GPRs 0 to 7. A CISC program may load one of its GPR's, such as EAX, with a data value, then switch to RISC mode, allowing the RISC program to read that value the CISC program placed in EAX merely by reading the RISC GPR 0. Since the RISC architecture defines GPR 0 as a regular GPR, many instructions can access this register without an explicit load from memory or register-to-register transfer. For example, a RISC add instruction could specify the value in GPR 0 that was loaded by the CISC program merely by identifying GPR 0 in one of the source fields in the RISC ADD instruction word. The result of the ADD may be written back to GPR 0 or any other GPR. Thus no explicit transfer is needed to access the CISC data by the RISC program.

The 6 CISC segment base address registers, ES Base to GS Base, may be implicitly read by the CISC program when generating an address. A RISC program may read or write these registers merely by specifying the corresponding GPR. If the CISC program required emulation code to load segment register FS Base with a base address, then the RISC emulation program merely has to identify GPR 12 in a RISC instruction to read or write this base address.

CISC programs, however, cannot freely access any RISC GPR except GPR 0 to 7. CISC programs may access GPR 8 to 13 in a restricted way, since these registers correspond to the CISC segment registers ES Base to GS Base. The CISC program may use a special segment override in the CISC instruction word to access one of these segment base registers when calculating an address. The CISC architecture imposes limitations on accessing these segment registers, making them useful for transferring address information between the RISC and CISC programs, but not useful for transferring data. The CISC program can only implicitly access these segment registers for address generation.

CISC mode programs have no access to RISC GPR 14 to 31, since there is no corresponding CISC register. However, emulation mode can access all 31 RISC registers, including the first 8 registers, which are the CISC GPR's, and the 6 CISC segment base registers. However, emulation mode can freely access the CISC segment base registers. Emulation mode executes RISC instructions, so the mechanism to transfer data and address information between CISC and emulation modes is similar to transfers between CISC and RISC modes as described above. The RISC instructions in emulation mode can implicitly access a CISC register by identifying the corresponding RISC GPR as a source in the RISC instruction word.

Emulation mode differs slightly from RISC mode. Normal RISC address checking and page fault handling is performed for accesses to most registers. However, when any of the 6 CISC segment registers, or GPR 14 or 15, are used to generate an address, the CPU uses x86-type address checking and x86 page fault handling, rather than the normal RISC address checking and page faulting routines.

GPR 14 and 15 are used in emulation mode as special emulation-mode segment base address registers. When emulation code generates an address using GPR 15 as one of the operands, no segment validity checking is performed at all, neither RISC nor x86 segment validity checking. Using GPR 15 allows for emulation code to generate an address without any segment checking. When emulation mode is entered, a 3-bit register is loaded with a pointer to one of the 6 CISC segment base registers (GPR 8 to GPR 13). The pointer value loaded is the segment used by the CISC instruction being emulated. This is normally the data segment register DS, or stack segment register SS, but a segment over-ride prefix attached to the CISC instruction could indicate that one of the other segment base registers be used. When register 14 is read the segment checking rules are determined by the last segment used by the last CISC mode memory access. The last segment used one of the segments in registers 8 to 13. This allows emulation mode to use CISC mode checking, but also have full control and access of the segment registers. Thus using GPR 14 allows the emulation code to generate an address using whichever CISC segment register has been used by the CISC instruction. This is a very powerful feature for emulation, saving dozens of instructions in the emulation routine to examine and decode the CISC instruction word to determine which segment register should be used.

Both registers GPR 14 and 15 are preferably loaded with the value zero so that they do not modify the address being generated. Thus using these registers in emulation mode alters the address checking being used. Emulation mode, although using RISC instructions, can have CISC address checking for certain addresses generated with the CISC segment base registers and the two emulation base registers GPR 14 and 15.

Some of the RISC GPR's may be used by emulation mode for particular purposes. For example, GPR's 14, 15, 30, and 31 may be used by emulation routines for address generation within emulation mode, and for various assist functions.

If RISC mode and emulation mode are to be both used at the same time on a system, then the RISC program should not overwrite the four special emulation mode registers, or the CPU hardware needs to provide two sets of registers for GPR 14, 15, 30, and 31, one set exclusively for RISC mode, with a second set exclusively for emulation mode, as well as other duplicated hardware. RISC-mode programs must also not overwrite GPR's 0-13, which are used for CISC mode architectural registers. Because general RISC user programs write these registers, and may not have a need for transferring data to a CISC program, a process or task switch from a CISC user program to a general RISC user program is handled as a normal task switch, with all registers being saved to a stack before the switch so that the values in the GPR's are not overwritten and lost.

The code segment base address is available in two separate registers: GPR 9 holds the CISC code segment base (CS Base) while the RISC count CTR register also holds this same code segment base. This is beneficial for modern pipelined and superscalar processors because the CTR/CS register can provide the branching unit with the code segment base address, while the GPR array also can provide the code segment base address to the execution unit. Thus the code segment base address may be provided from two separate registers to two separate units within the processor. Since these units are often separated, having the separate registers can save the delay in transferring the code segment from the GPR's to the branching unit. As the code segment base address is frequently used in address calculations, having it in two separate locations is useful, effectively doubling the available bandwidth for supplying this base address.

Merging the GPR's together with the CISC GPR and segment registers provides a very efficient and clean way of transferring address and data between CISC and RISC programs and emulation programs. Normal architectural features are used to access and transfer data. Data can be accessed explicitly by specifying the corresponding GPR as the source in the instruction word.

FLOATING POINT DATA REGISTERS COMBINED

Just as the general-purpose registers for integer data are combined for both RISC and CISC integer instructions, the floating point data registers are also combined for RISC and CISC floating point instructions. The advantages of allowing programs from both RISC and CISC instruction sets to access the same data registers can thus apply to either or both integer and floating point data formats. The more complex CISC floating point instructions can be emulated with several simpler RISC floating point instructions which operate on the same set of floating point data registers and set flags in the same floating point status register. Thus switching between instruction sets can be accomplished quickly without saving the architectural registers out to main memory.

Indeed, emulation of the more complex floating point instructions may be more beneficial than emulation of simpler integer instructions because of the extreme complexity of some of the complex floating point instructions. The more complex CISC floating point instructions include square root (FSQRT), and transcendental operations such as sine (FSIN), cosine (FCOS), partial tangent and arc-tangent (FPTAN, FPATAN), and several logarithmic operations. The complexity of these floating point instructions is much greater than that of simpler integer instructions. Thus emulation may be more desirable for floating point instructions than for integer instructions. The invention allows emulation to be performed with a second instruction set, such as a highly-optimized RISC instruction set.

PAIRING OF RISC FLOATING POINT DATA REGISTERS FOR CISC EXTENDED PRECISION

While integer data formats are typically 32-bits in width or size, floating point formats include more bits of precision by having mantissas of 53 or 64 bits. The range of these floating point formats is also increased by having exponents of 8, 11, or 15 bits. Standard or single-precision floating point uses 32 bits, with an 8-bit exponent and a 24-bit mantissa. The double-precision floating-point format uses 64 bits, with an 11-bit exponent and a 53-bit mantissa. RISC includes both single and double precision floating point formats. CISC also includes these formats, in addition to a third format, extended precision. Extended precision uses 80 bits, with a 15-bit exponent and a 65-bit mantissa, including one sign bit. The exponent is biased for single and double precision formats so a separate sign bit for the exponent is not needed.

If the 80-bit extended precision format is supported for CISC, then the 64-bit RISC floating point data registers could be extended to 80 bits. However, this adds expense as 16 bits of register cells must be added to each floating point data register. Since the x86 CISC architecture defines only eight floating point registers, one solution is to extend to 80 bits just eight of the thirty-two RISC floating point data registers.

The inventors have realized than none of the 32 RISC floating point data registers need to be extended to 80 bits. Instead, two 64-bit RISC registers may be paired to produce an 80-bit CISC format. Since only 8 registers are needed for CISC instructions, pairing two RISC registers for each CISC register requires 16 of the 32 RISC floating point data registers. Thus RISC programs still have 16 unused floating point data registers, while 16 registers are paired for the 8 CISC 80-bit-format registers.

FIG. 6 is an implementation of RISC floating point data register pairing for CISC extended precision. The merged CISC & RISC floating point data register file 60 includes 32 floating point registers, each having 64 bits. These 64 bits in each register are divided into 53 mantissa bits and 11 exponent bits for double precision.

When the CISC 80-bit extended precision is needed, 64-bit RISC registers are paired, which provides ample bits (128 bits) for the 80-bit format. The additional 48 bits are unused, although other information such as full, empty, or stack information could be stored in these 48 unused bits. The first eight registers are paired with the second eight registers to form the eight CISC 80-bit registers in the stack. For example, register 0 is paired with register 8 register 1 with register 9, register 7 with register 15.

FIG. 6 shows a read from the first register in the 8-register stack. Register 0 provides 64 bits, including 53 mantissa bits and 11 exponent bits. The paired register, register 8, provides 4 more bits of the exponent and 11 more bits of the mantissa. The 4 additional exponent bits are stored in the upper 4 bits of register 8, while the 11 additional mantissa bits are stored in the lowest 11 bits of register 8. The 4 additional exponent bits from register 8 are appended to the top (left, or most-significant) of the exponent bits from register 0, while the 11 additional mantissa bits from register 8 are appended to the bottom (right, or least-significant) of the 53 mantissa bits from register 0. This arrangement minimizes the physical routing of metal traces on a semiconductor chip, thus reducing cost. The final 80-bit floating point operand is assembled, and the leading one of the mantissa is inserted and/or generated before being input to floating point adder 64, or another floating point operation unit such as a multiplier or logic unit (not shown). A second operand is assembled from two other paired floating point registers in a manner similar to that shown for registers 0 and 8.

Since the x86 CISC architecture defines only two input operands, four 64-bit read ports are needed to floating point data register file 60. Since PowerPC™ RISC defines 3 input operands, one additional read port is needed, for a total of four read ports. Likewise, two 64-bit write ports are needed to floating point data register file 60 to support the writing of an 80-bit result. FIG. 7 shows floating point data register file 60 having two write and four read ports. Multi-port register files are well-known in the art and can be constructed from memory cells with multiple ports.

SHARED REGISTER ARCHITECTURE

FIG. 8 is a diagram of the shared registers in the dual-instruction set processor. The CISC EFLAGS register and the RISC CR register are combined into a single 32-bit CP/EFLAGS register 40 that can be accessed by CISC user programs and RISC user programs and emulation code. The CISC code segment base address register and the RISC count CTR register are merged to a single CS/CTR register 42, also accessible by CISC user programs and RISC user programs and emulation code. The RISC system save/restore (SRR0) register, which normally holds the address to return to after an interrupt has been processed, also holds the return address when emulation code was called. Thus SRR0 can hold a RISC address or a CISC address. The RISC or CISC user can indirectly load SRR0 register 44 by causing emulation code to be entered, but cannot directly access SRR0 register 44. However, RISC supervisor code and emulation code has full access to SRR0 register 44.

The RISC link register, which is used to hold a branch address, is combined with a CISC register that holds the instruction address of the most recent floating point instruction. This merged FP-IP/LR register 46 is also indirectly accessible by CISC programs because they cannot directly read or write it, but can only load it by executing a floating point instruction. RISC and emulation programs can freely access this merged FP-IP/LR register 46. The 32 general-purpose registers from RISC are merged with the 8 GPR's and 6 segment base registers from the CISC architecture into merged GPR's 48. Four of the RISC GPR's are used by emulation code for special uses, although emulation code can access all RISC and CISC registers.

Data registers for numbers in floating-point format are also merged into one set of floating-point data register file 60 for both RISC and CISC programs. Status and control bits for floating point operations in both RISC and CISC modes are combined into a single floating point status and control register FPSCR register 52.

ALTERNATE EMBODIMENTS

This improvement relates to a central processing unit (CPU) for a dual-instruction set architecture. While the detailed description describes the invention in the context of the PowerPC™ reduced instruction set computer (RISC) and the x86 complex instruction set computer (CISC), it is contemplated that the invention applies to other instruction sets besides PowerPC™ and x86, and to more than two instruction sets, and to architectures besides RISC and CISC, without departing from the spirit of the invention. The exact number of bits in each register may likewise be varied by persons skilled in the art without departing from the spirit of the invention, although architecture compatibility may be lost.

The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. 

We claim:
 1. A shared register system for a dual-instruction-set floating point processor, the shared register system comprising:a shared floating point register for storing information to be transferred between a first program comprised of floating point instructions from a CISC instruction set and a second program comprised of floating point instructions from a RISC instruction set, the CISC instruction set having a first encoding of operations to opcodes, the RISC instruction set having a second encoding of operations to opcodes, the first encoding of operations to opcodes being substantially independent from the second encoding of operations to opcodes; first means, coupled to the shared floating point register, for accessing the shared floating point register from the CISC instruction set, the first means writing information into the shared floating point register responsive to a first subset of instructions from the CISC instruction set; and second means, coupled to the shared floating point register, for accessing the shared floating point register from the RISC instruction set, the second means reading information from the shared floating point register responsive to a second subset of instructions from the RISC instruction set, whereby information is transferred from the first program to the second program using the shared floating point register.
 2. The shared floating point register system of claim 1 wherein the shared floating point register is in a plurality of floating point data registers in the dual-instruction-set floating point processor, each register in the plurality of floating point data registers for storing a mantissa portion and an exponent portion of a number represented in a floating-point format.
 3. The shared floating point register system of claim 2 wherein the plurality of floating point data registers include eight stack-accessible CISC floating point data registers.
 4. The shared floating point register system of claim 2 wherein the plurality of floating point data registers store floating point data in a RISC format, the shared register system further comprising:pairing means for accessing a pair of floating point data registers and for combining data from both paired registers to produce an operand in an extended-precision format, whereby an operand in the extended-precision format is accessed by pairing two registers for storing floating point data in the RISC format.
 5. The shared floating point register system of claim 4 wherein the paired registers comprise:a first register having a first exponent portion and a first mantissa portion, the first exponent portion and the first mantissa portion containing sufficient exponent and mantissa bits to store a number in a double-precision floating point format; a second register having an extended exponent portion and an extended mantissa portion; wherein the pairing means comprises: exponent-extension means, receiving the first exponent portion from the first register and receiving the extended exponent portion from the second register, for prefixing the extended exponent portion to the first exponent portion, wherein the extended exponent portion comprises the most-significant bits while the first exponent portion comprises the least significant bits of an exponent for the operand in the extended-precision format; mantissa-extension means, receiving the first mantissa portion from the first register and receiving the extended mantissa portion from the second register, for appending the extended mantissa portion to the first mantissa portion, wherein the extended mantissa portion comprises the least-significant mantissa bits while the first mantissa portion comprises the most significant mantissa bits of a mantissa for the operand in the extended-precision format, whereby the pairing means prefixes exponent bits but appends mantissa bits from the second register to form the operand in the extended precision format.
 6. The shared floating point register system of claim 1 wherein the RISC instruction set is a PowerPC™ instruction set, and the CISC instruction set is an x86 instruction set.
 7. The shared floating point register system of claim 1 wherein the shared floating point register comprises a first flags field for storing first flags implicitly set by floating point operations encoded by opcodes in the first subset of instructions from the CISC instruction set, the first flags also implicitly set by floating point operations encoded by opcodes in a third subset of instructions from the RISC instruction set, the second means for accessing the shared floating point register from the RISC instruction set writing information to the shared floating point register in response to instructions from the third subset of instructions from the RISC instruction set.
 8. The shared floating point register system of claim 7 wherein the first flags field in the shared floating point register is implicitly read by first instructions having opcodes encoding conditional branch operations, and wherein the second flags field in the shared floating point register is implicitly read by second instructions having opcodes encoding conditional branch operations.
 9. The shared floating point register system of claim 8 wherein the first flags include a zero flag indicating that a floating point operation had a zero-valued result.
 10. The shared floating point register system of claim 9 wherein the first flags include an exception flag indicating that an exception occurred when executing a floating point instruction.
 11. The shared floating point register system of claim 10 further comprising:control bits in the shared floating point register, the control bits for enabling reporting of various kinds of floating point exceptions; exception detection means for detecting various kinds of floating point exceptions when a floating point instruction is executed; exception disabling means, responsive to the control bits in the shared floating point register, for disabling the exception detection means and preventing the reporting of an exception and setting the exception flag in the first flags, whereby the control bits determine if the exception flag in the first flags is set when the exception occurs.
 12. The CPU of claim 11 wherein the exception is selected from the group consisting of an overflow exception, and underflow exception, a divide-by-zero exception, and a loss-of-precision exception.
 13. A floating point unit (FPU) for executing first instructions from a first instruction set and for executing second instructions from a second instruction set, the first instructions having a first field for specifying a destination floating point register on the FPU, the second instructions having a second field for specifying a source floating point register on the FPU, the FPU comprising:a first instruction decoder, receiving the first instructions from the first instruction set, the first instruction decoder providing decoded first instructions; a second instruction decoder, receiving the second instructions from the second instruction set, the second instruction decoder providing decoded second instructions; a floating point execution unit for executing first floating point instructions and for executing second floating point instructions, the floating point execution unit receiving decoded first instructions from the first instruction decoder, the floating point execution unit receiving decoded second instructions from the second instruction decoder; and a plurality of floating point registers on the FPU, a selected register in the plurality of floating point registers being written to by the floating point execution unit when the floating point execution unit receives a decoded first instruction, the selected register specified by the first field for specifying a destination register on the FPU, the selected register in the plurality of floating point registers being read from by the floating point execution unit when the floating point execution unit receives a decoded second instruction, the selected register specified by the second field for specifying a source floating point register on the FPU, whereby data may be transferred from a first instruction to a second instruction via the selected register.
 14. The FPU of claim 13 wherein the first instruction set has a first encoding of operations to opcodes, the second instruction set has a second encoding of operations to opcodes, the first encoding of operations to opcodes being substantially independent from the second encoding of operations to opcodes.
 15. The FPU of claim 14 wherein the second instruction set is a reduced instruction set computer (RISC) instruction set and the first instruction set is a complex instruction set computer (CISC) instruction set.
 16. A central processing unit (CPU) for executing first instructions from a first instruction set and for executing second instructions from a second instruction set, the CPU comprising:a first instruction decoder, receiving the first instructions from the first instruction set, the first instruction decoder providing decoded first instructions; a second instruction decoder, receiving the second instructions from the second instruction set, the second instruction decoder providing decoded second instructions; an execution unit for executing first instructions and for executing second instructions, the execution unit receiving decoded first instructions from the first instruction decoder, the execution unit receiving decoded second instructions from the second instruction decoder; and a condition code register comprising a first condition code and a second condition code, the first condition code being set by the execution unit when the execution unit receives a decoded first instruction and an arithmetic operation is executed, the second condition code being set by the execution unit when the execution unit receives a decoded second instruction and an arithmetic operation is executed, the first condition code being read by the execution unit when the execution unit receives a decoded first instruction having a first opcode indicating that the first condition code be read; the first condition code also being read by the execution unit when the execution unit receives a decoded second instruction having a second opcode indicating that the first condition code be read; the second condition code being read by the execution unit when the execution unit receives a decoded second instruction having a third opcode indicating that the second condition code be read; a floating point status and control register, having control bits for enabling detection of floating point exceptions by the execution unit, the floating point status and control register further having status bits for indicating when an enabled exception has been detected by the execution unit when executing a floating point instruction in the first instruction set or in the second instruction set, whereby the first condition code set by execution of the first instruction set may be read by the first instruction set or the second instruction set.
 17. The CPU of claim 16 wherein the first opcode designates a floating point operation that implicitly writes the first condition code, the second opcode and the third opcode encoding operations for a conditional branch operation that reads the condition code register to determine if a branch is taken.
 18. The CPU of claim 17 wherein the first instruction set has a first encoding of operations to opcodes, the second instruction set has a second encoding of operations to opcodes, the first encoding of operations to opcodes being substantially independent from the second encoding of operations to opcodes.
 19. The CPU of claim 16 wherein the enabled exception is selected from the group consisting of an overflow exception, and underflow exception, a divide-by-zero exception, and a loss-of-precision exception. 